Δεν είμαι τόσο μεγάλος όσο ο παππούς μου.

Δεν is the normal word for not before a verb in Greek.

So:

• είμαι = I am
• δεν είμαι = I am not

A common beginner question is why not όχι.
Greek usually uses:

• δεν to negate verbs
• όχι for no, or for emphasis/contrast

So in this sentence, Δεν είμαι... is exactly what you expect for I am not...

είμαι is the 1st person singular form of the verb to be.

So:

• είμαι = I am

Greek often leaves out the subject pronoun εγώ because the verb form already shows the subject.

That is why Greek says:

• Δεν είμαι... = I am not...

and usually not:

• Εγώ δεν είμαι...

unless the speaker wants extra emphasis on I.

Because Greek usually does not need subject pronouns when the verb ending already makes the subject clear.

Here, είμαι already means I am, so εγώ is unnecessary.

Compare:

• Είμαι κουρασμένος. = I am tired.
• Δεν είμαι τόσο μεγάλος... = I am not as old/big...

You can add εγώ for emphasis, contrast, or clarification, but it is not normally required.

This is the standard Greek pattern for as ... as.

So:

• τόσο μεγάλος όσο... = as big/old as...
• δεν είμαι τόσο μεγάλος όσο... = I am not as big/old as...

It works as a pair:

• τόσο = so / as much / that much
• όσο = as

Together they create the comparison.

A useful pattern to remember is:

• τόσο + adjective + όσο + noun/pronoun

Example:

• Είναι τόσο ψηλός όσο ο αδερφός του.
  = He is as tall as his brother.

It can mean either, depending on context.

μεγάλος literally means big or large, but when talking about people, it can also mean grown-up, older, or old.

In this sentence, with ο παππούς μου (my grandfather), the natural interpretation is usually old/older in age:

• I am not as old as my grandfather.

But depending on context, it could also be understood more literally as big.

This is very common in Greek: the same adjective can cover both ideas, and context tells you which meaning is intended.

Because adjectives in Greek must agree with the noun or person they describe in gender, number, and case.

Here, μεγάλος is:

• masculine
• singular
• nominative

So this version suggests the speaker is male.

If a woman were saying the same sentence, she would normally say:

• Δεν είμαι τόσο μεγάλη όσο ο παππούς μου.

If the adjective described a neuter noun, it would be μεγάλο.

So the ending changes depending on who or what is being described.

Greek often uses the definite article with family members and possessive expressions.

So:

• ο παππούς μου = my grandfather
• literally, something like the grandfather of me

This is very normal Greek grammar.
English says my grandfather without the, but Greek usually includes the article:

• ο αδερφός μου = my brother
• η μητέρα μου = my mother
• ο φίλος μου = my friend

So the article here is not extra; it is the natural Greek way to say it.

μου means my here.

More specifically, it is the weak possessive form of the pronoun I / me, historically related to the genitive.

So:

• ο παππούς μου = my grandfather
• literally, the grandfather of me

This pattern is extremely common in Greek:

• το βιβλίο μου = my book
• η φίλη μου = my friend
• το σπίτι μου = my house

So μου is the possessive element meaning my.

No change is needed here because ο παππούς μου is the noun being compared with the subject, so it stays in the nominative.

The structure is basically:

• I am not as old as my grandfather is

Greek does not need to repeat the second verb είναι, but the comparison still works as if it were there.

So:

• Δεν είμαι τόσο μεγάλος όσο ο παππούς μου.

is understood as:

• I am not as old as my grandfather is.

That is why ο παππούς μου appears in the nominative form.

Greek word order is more flexible than English word order, but not everything can move freely.

The normal order here is:

• Δεν είμαι τόσο μεγάλος όσο ο παππούς μου.

This is the most natural and neutral version.

You may see small variations for emphasis, but the basic comparison pattern τόσο ... όσο should stay clear.

For a learner, the safest choice is to keep this standard order.

Not usually, if she is talking about herself.

A female speaker would normally change the adjective to the feminine form:

• Δεν είμαι τόσο μεγάλη όσο ο παππούς μου.

The rest of the sentence stays the same.

So the adjective changes to match the speaker:

• male speaker: μεγάλος
• female speaker: μεγάλη

Yes. τόσο ... όσο is a very common and useful comparison pattern.

You can use it with many adjectives:

• τόσο ψηλός όσο = as tall as
• τόσο γρήγορος όσο = as fast as
• τόσο έξυπνος όσο = as smart as

And in the negative:

• Δεν είμαι τόσο ψηλός όσο αυτός.
  = I am not as tall as him.

So this sentence is a great model to learn and reuse.

You could keep the same sentence and stress ο παππούς μου in speech.

Greek also allows some word-order variation for emphasis, but for learners the easiest and safest method is just vocal emphasis:

• Δεν είμαι τόσο μεγάλος όσο ο παππούς μου.

If you want to sound natural, the standard structure is still best.
The important thing is that the comparison remains clear: τόσο ... όσο.